CONDENSED HISTORY OF LIGHTING
Figure courtesy of Jeff Tsao; a version of this figure was published
in IEEE Circuits and Devices Vol 20, No 3, pp 28-37,
Lighting technologies are substitutes for sunlight in the 425-675 nm spectral region where sunlight is most concentrated and to which the human eye has evolved to be most sensitive. The history of lighting can be viewed as the development of increasingly efficient technologies for creating visible light inside, but not wasted light outside, of that spectral region.
A 200-year perspective on that history is shown in the figure above. The left axis indicates luminous efficacy, in units of lumens (a measure of light which factors in the human visual response to various wavelengths) per watt. The right axis indicates the corresponding power-conversion efficiency for a tri-LED tri-color white light source with moderate color rendering (CRI=80) and relatively warm color temperature (CCT=3900K). For such a source, 400lm/W would correspond to 100% power-conversion efficiency.
The three traditional technologies are fire; incandescence; and fluorescence and high-intensity discharges (HID). These three traditional technologies have all made significant progress over the past 200 years, but appear to be saturating at efficiencies in the 1-25% range.
A new, fourth technology is solid-state lighting. In principle, the technology is simple: electrons and holes are injected into a forward-biased semiconductor p-n junction; they recombine creating photons; the resulting photons are extracted from the chip; then the photons are either mixed with different-color photons from other LEDs, or are energy down-converted into a distribution of colors using phosphors or other down-conversion materials, with the colors chosen so as to create the appearance of white.
In practice, there are losses at every step of the way,
and efficiently creating white light from semiconductor materials with
band-gaps that span the visible spectrum is extremely challenging.
Nevertheless, great strides are being made, and SSL-LED technology is
currently on a very rapid improvement curve, particularly the red, green and blue monochrome lamps necessary for white light.